1. Field of the Invention
The present invention relates to an analytical equipment and method for the selective detection of individual oxygenated compounds in complex organic mixtures, specially for fuels of the recent type that use oxygenated compounds, instead of lead, to increase their antiknock value (octane rating).
The already known analytical equipments of the type considered in the present invention are based on the specific response of oxygenates substances to the methods of flame ionization (O-FID) detection and comprise, downstream an equipment of chromatographic separation, a cracking reactor constituted by a capillary tube through which the gas under test flows, said tube being made of a platinum and rhodium alloy that is heated to high temperatures as well as, downstream, a catalytic hydrogenation microreactor and a flame ionization detector (FID).
In the cracking reactor, which is directly connected with the capillary column of chromatographic separation, the cracking reaction takes place, during which the individual oxygenated compounds, selectively eluted in sequence by the capillary column, are transformed into carbon monoxide according to the known reaction: EQU C.sub.n H.sub.m O.sub.x .fwdarw.x CO+m/2H.sub.2 +(n-x)C
while hydrocarbons constituting the organic matrix are converted into carbon and hydrogen.
As known (see Werner Schneider et al. "Journal of Chromatography" 245, 1982, 71-83; G. R. Verga and A. Sironi "Journal of High Resolution Chromatography & Chromatography Communications" Vol. 11, March 83, pages 248-252) said reaction takes place, in the equipments of the prior art, at temperatures of 1100-1300 degrees C., since the presence of a carbon layer (under the form of graphite) on the internal walls of the platinum/rhodium capillary tube is required for the correct performance of each analysis. The absence of said arrangement could involve, besides the already mentioned reaction, other reactions that would generate compounds different from the desired ones (for instance CO.sub.2 instead of CO) and, consequently errors or in any case lack of analytical precision.
In known equipments the carbon layer on the walls of the platinum/rhodium capillary tube is obtained by continuously enriching the carrier gas with some parts per million (ppm) of hydrocarbon, by means of an appropriate doping system, or by injecting into the equipment a suitable quantity of hydrocarbon before each analytical cycle.
Furthermore, in the time intervals between a series of analyses and the subsequent one, the cracking reactor is never brought to room temperature but kept at temperatures not going below 700 degrees C.
This precaution is necessary in order to avoid long idle times between one analysis and the following one and to avoid considerable sudden changes of temperature in the cracking reactor that would decrease the operating life of the capillary tube.
High operating and stand-by temperatures of the cracking reactor as well as the need of introducing a carbon excess cause a gradual deterioration of the platinum/rhodium capillary tube, that therefore must be replaced every 1000 hours of operation on the average: the replacement of the capillary tube involves dead times and outstanding costs, mainly because of the high cost of rhodium.